Fill-level measurements are currently performed in a large number of industrial applications. In the classic fill level measurement, there is located in the container a single fill substance, whose fill level is registered by means of a fill-level measuring device.
Besides that, there are a number of applications, in the case of which there is in the container not a single fill substance, but, instead, two different fill substances. When two different fill substances with different specific weights are present in a container, then that fill substance, which has the smaller specific weight, lies at equilibrium conditions on the fill substance with the greater specific weight. Two completely separated layers form. The boundary between the two fill substances is referred to as the interface.
Interfaces occur, for example, in the petrochemicals industry, where they are formed e.g. by water and hydrocarbons, e.g. oil. A further example is the foods industry, where interfaces form, for example, in fat separators.
If two different fill substances are present in the container, then so called interface measurements are performed.
Interface measurements represent a special form of fill-level measurements, and serve to determine the position of the interface in the container and/or the fill levels of the two fill substances in the container. The term, fill level, referred to the height, which a layer of a particular fill substance assumes in the container. Interface measurements are used in many fields of industrial measurements technology, in order to ascertain the amounts, especially volume and/or weight, of individual fill substances located in the container.
Interface measurements are performed with fill-level measuring devices working according to the travel time principle, examples being the products bearing the marks LEVELFLEX and MIKROPILOT of the Endress+Hauser company.
In such case, an electromagnetic signal is sent into the container. This happens e.g. in that an electromagnetic signal is radiated by means of an antenna in the form of free radiation into the container, or in that a waveguide is inserted in the container, on which the electromagnetic signal is sent in the form of guided electromagnetic waves into the container. A part of these electromagnetic signals is reflected on the fill substance upper surface of the upper fill substance. A further part of the signals penetrates the upper fill substance and is reflected on the interface between the two fill substances. The measuring device receives an echo signal formed by these reflections and ascertains therefrom, on the one hand, the travel time, which passes between the transmitting of the signal and the receipt of the echo arising by the reflection on the fill substance upper surface of the upper fill substance and, on the other hand, the travel time, which passes between the transmitting of the signal and the receipt of the echo signal arising by the reflection at the interface, or on the fill substance upper surface of the lower fill substance.
The determining of these two travel times occurs on the basis of known travel time, measuring methods. In the case of guided electromagnetic signals, for example, time domain reflectometry is used. In such case, for example, according to the method of the guided microwave, a high-frequency pulse is transmitted along a Sommerfeld waveguide, a Goubau waveguide or a coaxial waveguide. If this electromagnetic signal meets a fill substance upper surface in the container, then at least one part of the signal is reflected back due to the impedance jump existing at this media boundary. The received signal amplitude as a function of the time represents the echo signal. Each value of this echo signal corresponds to the amplitude of an echo reflected at a certain distance from the transmitting, and receiving, element. The echo signals have marked maxima, which correspond to portions of the electromagnetic signals reflected on one of the fill level upper surfaces. From the time difference between the transmitting of the electromagnetic signal and the receipt of the maxima, the sought travel time and therewith also the position of the respective fill substance upper surface in the container is ascertained.
Used in connection with electromagnetic signals radiated in the form of free radiation into the container are the frequency modulation, continuous wave radar method (FMCW method) and the pulse radar method. Both methods are known in fill level measuring technology and, consequently, not explained here in detail.
On the basis of the structural dimensions of the measuring arrangement, especially the installed height of the fill level measuring device in reference to the container, and on the basis of the propagation velocities of the electromagnetic signals in a medium, e.g. air, located above the upper fill substance, and in the upper fill substance, from these two travel times, the fill levels of the two fill substances in the container and the total fill height present in the container can be calculated.
The structural dimensions of the measuring arrangement and the propagation velocity of the electromagnetic signals in the medium located above the upper fill substance are, as a rule, known. The knowledge of the propagation velocity of the electromagnetic signals in the upper fill substance can be calculated, for example, on the basis of the dielectric constant of this fill substance, or ascertained experimentally. If the dielectric constant the upper fill substance is not known and the propagation velocity cannot, e.g. for administrative reasons, be ascertained experimentally, then no interface measurement is possible in the described manner.
Interface measurements with fill level measuring devices working according to the travel time principle deliver very good results, when the media boundaries on the upper surface of the upper fill substance and between the two fill substances are sharply defined. This is especially the case, when the fill substances in the container are resting. There are, however, a large number of applications, in the case of which the fill substances, e.g. from supplying or draining of fill substance, are never left to themselves long enough to reach equilibrium conditions, in which the two fill substances lie completely separated from one another in two layers with surfaces sharply defined relative to one another. Foam or emulsion can form in the container. While at sharply defined media boundaries, an impedance jump is present, which effects a partial reflection of the signal at such a media boundary, foam formation and/or emulsion formation lead, as a rule, to the fact that there is no narrowly localized impedance jump marked by the media boundary, but, instead, a continuous transition is present. This can in the worst case lead to the fact that the echo signal, in the case of the presence of foam and/or emulsion has only one or no marked maximum, whose travel time can be measured. If there is only one maximum, then, on the basis of the echo signal, it is no longer recognizable, whether it relates to a reflection of the signal on the fill substance upper surface of the upper or the lower fill substance. A interface measurement is in such case no longer possible.
Interface measurements can, to a limited extent, also be performed with capacitive fill level measuring devices, such as are available, for example, from the assignee. Such a method is described, for example, in the 1990 published book: FILL LEVEL MEASURING TECHNOLOGY IN THEORY AND PRACTICE, by Wlm. van de Kamp, in Section 3.6.
For this, a capacitive probe is inserted into the container and a capacitance of a capacitor formed by the probe and the container wall surrounding it measured. The measured capacitance corresponds to the sum of a base capacitance of the empty container, the product of a fill substance specific, capacitance increase factor of the upper fill substance and its fill level and the product of a fill substance specific, capacitance increase factor of the lower fill substance and its fill level.
This method is, however, only applicable, when the two fill substance specific, capacitance increase factors and the total fill height of the two fill substances present in the container are known. The latter component corresponds to the sum of the fill level of the upper and the fill level of the lower fill substance and must either be known from application-specific conditions or separately measured.
There are classical fill level measuring devices known for the measuring of a fill level of a single fill substance contained in a container, in the case of which the travel time measurement principle is combined with the capacitive measuring principle in a measuring device. An example of this is the apparatus described in German Patent, DE 100 37 715 A1 of the assignee for measuring a fill level of a single fill substance present in a container. The apparatus includes a probe, which can be operated selectively as capacitive probe of a classic capacitive fill-level measuring device as well as also as waveguide of a classic fill level measuring device working according to the travel time principle.
A further example is described in German Patent DE-A1195 10 484. This application describes a fill level measuring device working according to the travel time principle with a waveguide, in the case of which in the waveguide a metal inner conductor is provided, which serves as capacitive probe. It is indicated, that by the combination of these two measuring principles in one measuring device as regards the classical fill level measurement a redundant system is provided, in the case of which the capacitive probe is used as an overfilling preventer.